In vivo acetylation of histones H3 and H4 leads to increased susceptibility of nuclear DNA to DNAase I and more rapid excision of nucleosomes containing the modified histones. Minor effects on core particle structure are observed. The chromatosome, a chromatin particle containing 160 base pairs of DNA and all the histones, has been isolated and studied. The particle is compact in shape, markedly stable to thermal denaturation, and highly resistant to DNAase I. Mapping experiments suggest that it contains an additional 10 base pairs of DNA on each end of the core particle 140 base pair length. Thyroid hormone (T3) binds to a limited number of sites in liver chromatin with high affinity. Other tissues differ from liver in having fewer sites and lowered stability. Initial experiments suggest that monomer nucleosomes do not bind the hormone. The carcinogen, benzpyrene diol epoxide interacts with HeLa cell core particles to covalently modify many proteins. Nonhistones are generally modified to a greater extent than the histones. A nonhistone protein of MW about 40,000 is modified more rapidly than any other component of the core particle.